Heat treatable four layer anti-reflection coating

ABSTRACT

A coated article includes a heat treatable (e.g., temperable) antireflection (AR) coating having four layers. The AR coating includes a layer adjacent the glass substrate having an index of refraction substantially matching that of the glass substrate, and having a compressive residual stress. In certain example embodiments, the coating may include the following layers from the glass substrate outwardly: stress-reducing layer/medium index layer/high index layer/low index layer. In certain example embodiments, depending on the chemical and optical properties of the high index layer and the substrate, the stress-reducing layer of the AR coating is selected to cause a net compressive residual stress and thus improve the overall performance of the antireflection coating when the coated article is heat treated.

This application is a divisional of application Ser. No. 12/929,481,filed Jan. 27, 2011, the entire disclosure of which is herebyincorporated herein by reference in this application.

Certain example embodiments of this invention relate to a coated articleincluding a heat treatable (e.g., temperable) anti-reflection coating,and/or a method of making the same. In certain example embodiments, aheat treatable (e.g., temperable) anti-reflection (AR) coating utilizesmaterials having a higher magnitude of compressive stress in connectionwith high index layer(s) that are prone to tensile stress upon heattreatment. These materials having a higher magnitude of compressivestress are utilized in order to help reduce the net tensile stress(e.g., reduce the tensile stress to a lower value of tensile stressand/or reduce the tensile stress to the point that the stress iscompressive rather than tensile) of the overall coating. In certainexample embodiments, the thicknesses and types of stress in each layermay be optimized in order to produce a heat treatable (e.g.,temperable), four layer AR coating.

BACKGROUND AND SUMMARY OF CERTAIN EXAMPLE EMBODIMENTS OF THE INVENTION

Anti-reflection (AR) coatings are known in the art. For example, ARcoatings in the visible range are widely used on glass in electronics,lighting, appliances, architectural, and display applications. However,in many of these applications, tempered or heat-strengthened glass maybe required. Tempering or heat strengthening of the glass is sometimesdone prior to the deposition of the AR coating to avoid unwanted changesin the optical, mechanical, or aesthetic quality of the coating as aconsequence of exposing the coating to the high temperatures requiredfor tempering and other forms of heat treatment. However, this “temperthen coat” method may be undesirable in certain circumstances. Thus, itwill be appreciated that there exists a need in the art for improved ARcoatings (e.g., temperable AR coatings) for coated articles such aswindows and the like.

In certain example embodiments, coatings may suffer from a lack ofdurability, particularly after heat treatment and/or thermal tempering,due to a residual net tensile stress that is too high. A large enoughnet tensile stress in a layer stack may cause aesthetic degradation ofthe coating (e.g., cracking), especially after a heat strengtheningand/or tempering process.

Those skilled in the art thus will appreciate that there also is a needfor improved durability in anti-reflection coatings

In certain example embodiments of this invention, there is provided amethod of making a heat treatable coated article comprising ananti-reflection coating, the method comprising: disposing astress-reducing layer over and contacting a glass substrate, thestress-reducing layer having an index of refraction that substantiallymatches an index of refraction of the glass substrate; disposing a layerhaving a medium index of refraction over the stress-reducing layer;disposing a layer having a high index of refraction over the mediumindex layer; disposing a layer having a low index of refraction over thehigh index layer, where the index of refraction of the high index layeris greater than those of the medium and low index layers, and the indexof refraction of the low index layer is less than that of the mediumindex layer; and wherein a net residual stress of the anti-reflectioncoating is compressive (e.g., following optional heat treatment).

In certain example embodiments of this invention, there is provided amethod of making a heat treatable coated article comprising ananti-reflection coating, the method comprising: disposing astress-reducing layer over and contacting a glass substrate, thestress-reducing layer comprising an oxide and/or nitride of silicon, andhaving an index of refraction that differs from an index of refractionof the glass substrate by no more than about 0.2; disposing a layercomprising silicon oxynitride over the stress-reducing layer; disposinga layer comprising an oxide of niobium over the layer comprising siliconoxynitride; disposing a layer comprising an oxide of silicon over thelayer comprising an oxide of niobium; and wherein the index ofrefraction of the high index layer is greater than those of the mediumand low index layers, and the index of refraction of the low index layeris less than that of the medium index layer, and wherein a net residualstress of the coating is compressive.

In certain example embodiments of this invention, there is provided aheat treatable coated article comprising: an antireflection coatingsupported by a major surface of a substrate, the substrate being heattreatable together with the antireflection coating, wherein theantireflection coating comprises, in order moving away from thesubstrate: a stress-reducing layer comprising a material having acompressive residual stress after heat treatment; a medium index layercomprising silicon oxynitride; a high index layer comprising an oxide ofniobium, and a low index layer comprising an oxide of silicon, whereinthe coating has a net compressive residual stress (e.g., following heattreatment).

In certain example embodiments of this invention, there is provided aheat treated coated article comprising: an antireflection coatingsupported by a major surface of a substrate, the substrate being heattreated together with the antireflection coating, wherein theantireflection coating comprises, in order moving away from thesubstrate: a stress-reducing layer comprising an oxide and/or nitride ofsilicon; a medium index layer comprising silicon oxynitride; a highindex layer comprising an oxide of niobium, and a low index layercomprising an oxide of silicon, wherein the coating has a netcompressive residual stress after heat treatment.

In certain example embodiments, the same or similar antireflectivecoatings may be provided on one or both major surfaces of the glasssubstrate.

The features, aspects, advantages, and example embodiments describedherein may be combined to realize yet further embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a three-layered antireflectioncoating.

FIG. 2 is a cross-sectional view of a four-layered antireflectioncoating according to certain example embodiments.

FIG. 3 is a cross-sectional view of a four-layered antireflectioncoating according to other example embodiments.

FIG. 4 is a cross-section view of a four-layered antireflection coatingaccording to still further example embodiments.

DETAILED DESCRIPTION OF CERTAIN EXAMPLE EMBODIMENTS OF THE INVENTION

Referring now more particularly to the accompanying drawings in whichlike reference numerals indicate like parts throughout the severalviews.

Certain example embodiments of this invention relate to a coated articleincluding an antireflective coating, and/or a method of making the same.In certain example embodiments, a heat treatable (e.g., temperable)anti-reflection (AR) coating is provided.

AR coatings with antireflective properties in the visible range arewidely used on glass in electronics, lighting, appliances,architectural, display applications, and other applications. Althoughtempering or heat strengthening of the glass is sometimes done prior tothe deposition of the AR coating to avoid unwanted changes in theoptical, mechanical, or aesthetic quality of the coating as aconsequence of exposing the coating to the high temperatures requiredfor tempering and other forms of heat treatment, there are drawbacksassociated with the “temper then coat” method under certain examplecircumstances. For example, tempering prior to coating may beundesirable for large area coaters. The final size of the tempered/heattreated substrate to be coated may be of a dimension that does notefficiently employ the large area coating capability, which is usefulwhen attempting to increase or achieve the high efficiencies possible byvirtue of high volume glass coating manufacturing techniques. Therefore,it will be appreciated that an antireflective coating (e.g., afour-layer AR coating) that can be tempered and/or heat treated whilepreserving its aesthetic quality and high chemical and mechanicaldurability after exposure to temperatures typically encountered intempering and/or heat treating environments would be advantageous.

Existing AR coatings may not be sufficiently heat treatable temperable)in certain example embodiments, e.g., in the sense that such coatingsmay not survive the tempering or heat strengthening process in a usableor desirable form. As one example, it is noted that some materialsutilized in AR coatings may have high tensile residual stress afterexposure to temperatures greater than, for example, 300 degrees C. Whenthe tensile residual stress of one layer is so high such that it resultsin a net tensile stress in the multilayer stack, this stress may besufficient to cause an aesthetic degradation of the coating. This and/orsimilar problems may, for example, result in the cracking of thecoating. Therefore, it may be advantageous to reduce the tensileresidual stress in a layer in an AR coating.

When a material is subjected to tensile stress, the material may sufferstretching or elongation. Accordingly, if too much tensile residualstress is present in a layer in a coating, the layer and/or coating maysuffer deformation or cracking in certain instances, which may cause theaforesaid degradation of the coating. On the other hand, compressivestress, when applied, acts toward the center of a material. Thus, when amaterial is subjected to compressive stress, the material is undercompression. Therefore, in certain example embodiments, it may be moredesirable for a coating to have a net compressive residual stress ratherthan a net tensile residual stress.

FIG. 1 is a cross-sectional view of an example coated article accordingto an example embodiment of this invention. The coated article of theFIG. 1 embodiment includes substrate 1 that supports heat treatable(e.g., temperable) AR coating 3. Substrate 1 is typically a glasssubstrate (e.g., clear, green, bronze, or blue-green glass substratefrom about 1.0 to 10.0 mm thick), but may be other materials in certainexample instances such as, for example, a polycarbonate or acrylicmaterial, a silicon wafer, etc. The AR coating 3 includes medium indexlayer 7, high index layer 9, and low index layer 11.

In certain example embodiments, layers having high indices of refractionare associated with a tensile residual stress induced during heattreatment. Therefore, in order improve the durability and/or otherproperties of a temperable anti-reflection coating comprising at leastone layer with a high index of refraction, it may be advantageous toreduce and/or offset the tensile stress of the high index layer incertain example embodiments.

Tables 1 and 2 illustrate the correlation between cracking of athree-layered AR coating and the net post-heat treatment residual stressin layer stacks after heat treatment at 550 degrees C. for approximately10 minutes in certain example embodiments. In the comparative exampleslisted in Tables 1 and 2, TiO_(x) has been used as the high indexmaterial. However, other example embodiments of the invention are not solimited.

In order to test the incidence of cracking, in Comparative Examples 1-5,layer stacks were deposited on the Sn side of a glass substrate. Inthese examples, the thicknesses of the silicon oxynitride and thesilicon oxide-based layers were kept substantially constant for eachexample, whereas the thicknesses of the high index layers (here,TiO_(x)) were varied within a range of 20 to 100 nm.

Based on these examples, in certain embodiments, cracking may not occurand/or the incidence of cracking may be reduced if the net stress in thelayer stack is zero or compressive. By convention, the compressivestress is denoted as negative, and tensile stress is denoted aspositive.

TABLE 1 Comparative Example SiO_(x) (nm) TiO_(x) (nm) SiO_(x)N_(y) (nm)Total Thickness 1a 84.9 111.7 70.2 266.8 2a 82.5 71.9 73.0 227.3 3a 82.546.1 73.0 201.6 4a 82.5 21.5 73.0 177.0 5a — — 72.4 —

TABLE 2 Compar- ative As-Coated Post HT (650° C., 10 min) Crack- ExampleΣ_(x), MPa Σ_(y), MPa Avg. Σ_(x), MPa Σ_(y), MPa Avg. ing? 1a −136.7−124.7 −130.7 204.9 191.4 198.2 Yes 2a −225.4 −214.9 −222.3 85.5 81.481.0 Yes −222.6 −226.2 70.1 87.1 3a −255.6 −251.0 −259.1 6.5 10.95 7.1No −268.5 −261.4 12.0 −1.0 4a −298.9 −291.0 −293.9 −97.4 −107.1 −101.1No −295.7 −290.0 −98.6 −101.3 5a −565.2 −562.8 −570.7 −324.7 −340 −343.2N/A −622.9 −531.7 −375.7 −332.1

As can be seen from Tables 1 and 2, when there is very little residualtensile stress and/or compressive stress as the net residual stress inthe layer stack after heat treatment, the coating may be less inclinedto crack in certain example instances. Accordingly, it would bedesirable to reduce net tensile stress of a layer stack, as this may inturn reduce the likelihood of the coating cracking in certain exampleembodiments.

Further, from Comparative Example 5a, for example, it can be seen thatthe net residual stress of a single layer comprising silicon oxynitrideis compressive. In fact, the stress of a layer comprising siliconoxynitride with a thickness as shown for Comparative Example 5a in theabove example has a very large compressive stress. In certain exampleembodiments, when paired with layers having less compressive residualstress and/or tensile stress, the overall coating may still exhibit anet compressive stress due the large compressive stress of the siliconoxynitride-based layer.

There are several approaches possible when attempting to reduce the nettensile residual stress in a layer stack. In certain exampleembodiments, the thickness of the high index layer(s) may be reduced,thereby reducing the total tensile stress in the layer stack. In otherexample embodiments, a high index layer having a lower post-heattreatment tensile stress may be used instead of a high index layer witha higher post-heat treatment tensile stress. In further exampleembodiments, the magnitude of compressive stress in the other layers maybe increased in order to further offset and/or balance the residualtensile stress in the high index layer. The aforesaid methods forreducing the net tensile residual stress may be used alone or incombination with other methods.

In certain example embodiments, if the thickness of the high index layeris too small, the optical performance of the coating may be compromised(e.g., some loss of color neutrality). This may be due in some instancesto a narrower spectral bandpass that results when the high indexmaterial is thinner. Thus, for optical reasons, it may be advantageousto provide an antireflection coating having a high index layer with aphysical thickness of from about 50 to 250 nm, more preferably fromabout 75 to 125 nm, even more preferably from about 80 to 120 nm, with anon-limiting example being at least about 100 nm.

Replacing a high index layer having a higher residual tensile stressafter heat treatment (e.g., TiO_(x)) with a high index layer having alower residual tensile stress after heat treatment and/or increasing themagnitude of compressive stress in the other layers of the stack may beadvantageous in that the durability of the coating after heat treatmentmay be improved in certain cases. It would be further advantageous tohave the means to theoretically predict the net residual stress ofseveral designed layer stacks, in order to identify materials and stackdesigns with the least likelihood of cracking after heat treatment.

In certain example embodiments, such theoretical prediction is possiblebased on the individual bending moments and curvatures of each layerand/or film in the coating.

More specifically, for sequentially deposited films, each filmintroduces a separate bending moment which results in a separatecurvature in certain cases. Since the bending moments are additive, soare the curvatures, in certain example embodiments.

From repeated application of Stoney's equation, the following equationis obtained:

$\begin{matrix}{{{{\Delta\left( \frac{1}{R_{1}} \right)} + {\Delta\left( \frac{1}{R_{2}} \right)} + \ldots + {\Delta\left( \frac{1}{R_{n}} \right)}} = {\frac{6\left( {1 - v_{s}} \right)}{E_{s}h_{s}^{2}}\left( {{\sigma_{1}h_{1}} + {\sigma_{2}h_{2}} + \ldots + {\sigma_{n}h_{n}}} \right)}},} & (1)\end{matrix}$where σ_(n) is the residual stress of the n^(th) layer, and h_(n) is thethickness of the n^(th) layer, and together these variables result in achange of curvature, which is Δ(R⁻¹ _(n)). v_(s) is the Poisson Modulus,and E_(s) is Young's Modulus. After further simplification of the aboveequation, the rule for the addition of stress within a layer stack isgiven by the following equation (Equation 2) if the stress of each layerfor a given thickness is known.

$\begin{matrix}{\sigma_{Net} = {\frac{{\sigma_{1}h_{1}} + {\sigma_{2}h_{2}} + \ldots + {\sigma_{n}h_{n}}}{h_{1} + h_{2} + \ldots + h_{n}}.}} & (2)\end{matrix}$

Table 3 below illustrates the validity of Equation 2 by comparing thecalculated net residual stress in the stack with the measured value foranother example stack. In certain example embodiments, the calculatedvalue may vary from the measured value by less than or equal to about20%, more preferably by less than or equal to about 15%, and in certainexemplary embodiments, by less than or equal to about 13%. It will beappreciated that this discrepancy between the calculated and measuredvalues in Table 3 is reasonably within the error of the stress valuesobtained from measurement in certain examples.

TABLE 3 Adjusted for thickness Σ_(Net), MPa Material Σ, MPa/nm h, nm σ₁= hΣ, MPa Calc. Meas. SiOx −1.37 102.8 −140.8 −69.6 −60 NbOx 3.35 105.3352.8 SiOxNy −3.95 106.2 −419.5

In the calculations illustrated in Table 3, a proportional relationshipbetween residual stress and layer thickness was assumed, and thedifference between calculated and measured values was about 13%.However, in other example embodiments, this dependence may not bestrictly proportional. In fact, in certain instances, the relationshipbetween residual stress and layer thickness may be better approximatedby including a first order component in the relationship. However, thismay not be necessary or desired in other example embodiments.

Table 4 illustrates experimentally determined values of residual stressmeasured on an Si wafer after heat treatment at 650 degrees C. for 10min. These values were normalized by layer thickness. In certainexamples, a proportional relationship between the residual stress andvarious physical thicknesses was assumed. The highlighted valuesindicate the normalized residual stress value(s) that will be used inthe calculations that follow.

TABLE 4 Deposition Parameters Heat Treated: P 650 degrees ° C., 10 min.Ex. Material O₂ A (kW) ISPD Pass h, nm σ_(avg), MPa Σ_(avg), MPa/nm 1bNbO_(x) 60 40 2.5 30 20 87.9 294 3.35 Avg 2b NbO_(x) 60 40 1 30 10 177.0422 2.38 3b SiO_(x),N_(y) 10 10 2.5 30 8 107.0 −423 −3.95 4bSiO_(x),N_(y) 10 10 2.5 30 8 106.2 −60 N/A NbO_(x) 60 40 2.5 30 26 105.3SiO_(x) 40 10 2.5 30 15 102.8 5b SiO_(x) G49 86.4 −119 −1.37 6bSiO_(x)N_(y) 72.4 −343 −4.74 7b SiO_(x)N_(y) 71.4 −227 −3.18 −3.96

Table 5 illustrates the optical performance and calculated post-heattreated stress values for the various thicknesses indicated above. Forthe calculation of the optical quantities, a refractive index of about1.75 was used for the silicon oxynitride-based layer, a refractive indexof above 1.48 for the silicon oxide-based layer, and a refractive indexof about 2.3 for the niobium oxide-based layer were used in thecalculation. The calculated stress values predict a risk of crackingafter heat treatment in certain example embodiments, despite thesubstitution of TiOx with a lower tensile stress material such as NbOx.

TABLE 5 Σ₁ σ₁ = hΣ Normal Material h, nm (MPa/nm) MPa σ_(Net) MPa Inc. RT SiOx 83 −1.37 −113.7 19.4 Y  0.46 98 NbOx 96 3.35 321.6 a* 8.3 −1.3SiOxNy 65 −3.95 −256.8 b* −13 0.6 SiOx 82 −1.37 −112.3 31.3 Y  0.53 97.9NbOx 96 3.35 321.6 a* 0.35 −0.93 SiOxNy 60 −3.95 −237.0 b* −5.2 0.22SiOx 80 −1.37 −109.6 50.7 Y  0.72 97.7 NbOx 97 3.35 325.0 a* −8 −0.57SiOxNy 53 −3.95 −209.4 b* 0.32 −0.02

Accordingly, in addition to and/or instead of replacing the high indexlayer with a layer comprising a lower tensile stress material, anadditional layer may be utilized.

In certain example embodiments, a fourth layer may be used in an ARlayer stack in order to reduce the net residual tensile stress of thelayer stack and/or increase the net residual compressive stress. Incertain examples, a four layer AR coating may comprise an index matchinglayer adjacent to the glass substrate. The index matching layer may havean index of refraction close to that of the glass substrate. Forexample, the layer may have an index of refraction from about 1.35 to1.65, more preferably from about 1.4 to 1.6, and most preferably fromabout 1.45 to 1.55. Further, the stress-reducing layer may comprise anindex of refraction differing from that of the glass substrate by nomore than about 0.2, more preferably no more than about 0.1, and mostpreferably no more than about 0.05. In certain non-limiting embodiments,the stress-reducing, index-matching layer may comprise silicon oxideand/or silicon oxynitride.

In certain example embodiments, this index matching layer may serve anadditional purpose of compensating for the tensile stress of the highindex layer. In other words, in certain cases, the index matching layermay also be a stress reducing layer. This index matching and/or stressreducing layer may comprise a material having an index of refractionthat substantially matches that of the glass substrate, and that alsohas a net compressive stress, in certain example embodiments.

FIG. 2 is a cross-sectional view of an example coated article accordingto an embodiment of this invention. The coated article of the FIG. 2embodiment includes substrate 1 that supports heat treatable (e.g.,temperable) anti-reflection (AR) coating 4. Substrate 1 is typically aglass substrate (e.g., clear, green, bronze, or blue-green glasssubstrate from about 1.0 to 10.0 mm thick), but may be other materialsin certain example instances such as, for example, a polycarbonate oracrylic material, a silicon wafer, etc. The AR coating 4 includes an“index-matching” and/or “stress-reducing” layer 5, medium index layer 9,high index layer 9, and low index layer 11.

In certain example embodiments, layer 5 may comprise a material with arefractive index substantially matching that of a glass substrate. Incertain cases, even if the refractive index of layer 5 does not matchthat of the glass substrate exactly, it still may have a refractiveindex sufficiently close enough such that any impact to opticalperformance can be reduced with slight modification to a thickness(es)of one or more other layers in the stack. Moreover, the thickness oflayer 5 may be increased in certain instances as necessary, e.g., to adda sufficient component of compressive stress to the layer stack afterheat treatment.

In certain example embodiments, layer 5 may comprise silicon oxideand/or silicon oxynitride. However, the invention is not so limited, andlayer 5 may comprise any material having a refractive index thatsubstantially matches that of the glass substrate. By “substantiallymatches,” it is meant that the refractive index of the layer is withinabout 0.2 of the refractive index of the glass substrate, morepreferably within about 0.1, and most preferably the difference is nogreater than about 0.05 or 0.04.

Furthermore, in certain example embodiments, layer 5 may have athickness of from about 50 to 300 nm, more preferably from about 60 to120 nm, and most preferably from about 60 to 100 nm. However, a layerhaving any thickness sufficient to turn the net stress of the coatinginto compressive stress without significantly degrading the opticaland/or physical characteristics of coating may be used in other exampleembodiments.

Layer 7 may be of or include a material having a “medium” index ofrefraction in certain example embodiments. The refractive index of layer7 may be lower than that of layer 11, but higher than that of layer 9.In certain example embodiments, the refractive index of layer 7 may alsobe higher than the refractive index of layer 5.

In certain example embodiments, layer 7 may have a thickness of fromabout 30 to 150 nm, more preferably from about 40 to 80 nm, and mostpreferably from about 50 to 70 urn, with an exemplary thickness rangebeing from about 53-65 nm.

Layer 7 may comprise silicon oxynitride in certain example embodiments;however, the invention is not so limited. Layer 7 may comprise anymaterial having a refractive index from about 1.6 to 2.0, morepreferably from about 1.65 to 1.95, and most preferably from about 1.7to 1.8 or 1.9.

Layer 9 may be of or include a material having a comparatively “high”index of refraction in certain example embodiments. The refractive indexof layer 9 may be greater than that of the other three layers making upthe AR coating. In certain example embodiments, layer 9 may have arefractive index of from about 2.0 to 2.6, more preferably from about2.1 to 2.5, and most preferably from about 2.2 to 2.4.

In certain example embodiments, layer 9 may have a thickness of fromabout 50 to 150 nm, more preferably from about 75 to 125 nm, even morepreferably from about 80 to 120 nm, and most preferably from about 85 to105 nm.

In other example embodiments, however, layer 9 may be thinned in orderto reduce the net tensile stress of the AR coating, e.g., such thatlayer 9 has a thickness of less than about 50 nm, or even less thanabout 75 am in some instances.

In further example embodiments, layer 9 may comprise a high indexMaterial having a lesser tensile stress value, before and/or after heattreatment. Layer 9 may comprise an oxide of niobium in some instances.In other instances, layer 9 may comprise an oxide of titanium. Infurther example embodiments, layer 9 may comprise any suitable, highindex material.

Layer 11 may be of or include a material having a “low” index ofrefraction. In certain example embodiments, layer 11 may have an indexof refraction that is lower than that of both layers 7 and 9. In somecases, layer 11 may even have an index of refraction lower than that oflayer 5. In certain examples, the refractive index of layer 11 may befrom about 1.3 to 1.6, more preferably from about 1.35 to 1.55, and mostpreferably from about 1.43 to 1.52.

In certain example embodiments, layer 11 may comprise silicon. In somecases, layer 11 may comprise silicon oxide and/or silicon oxynitride.Layer 11 may have a thickness of from about 40 to 200 nm, morepreferably from about 50 to 110 am, and most preferably from about 60 to100 nm, with an example thickness being around 80 nm.

In some example embodiments, layers 5 and 11 may comprise similarthicknesses, and/or may even comprise substantially the same thickness.Layers 5 and 11 may differ from each other in thickness by no more thanabout 15 nm, more preferably no more than about 10 nm, and mostpreferably no more than about 5 nm, according to certain exampleembodiments.

An example embodiment of a coated article made according to FIG. 2 isillustrated in Table 6. As can be seen from Table 6, with the inclusionof an additional index-matching/stress-reducing layer, a coatingincluding an additional layer with a higher magnitude of compressivestress has a lower overall net stress. In fact, in Example 1c, athree-layered AR coating, the net stress is tensile, but in Example 2c,the four-layered AR coating, the net stress is compressive. Thus, incertain example embodiments, a coating made with an additional layerhaving a greater compressive stress may be more durable than athree-layered AR coating.

TABLE 6 Σ₁ Nor- h, (MPA/ σ₁ = hΣ, σ_(Net) mal Material nm nm) MPa MPaInc. R T Ex. 1c SiOx 79 −1.37 −108.2 32.8 Y  0.61 97.8 NbOx 94 3.35314.9 a* −.51 −0.9 SiOxNy 55 −3.95 −217.3 b* −0.12 −1.01 Ex. 2c SiOx 78−1.37 −112.3 −4.9 Y  0.71 97.7 NbOx 90 3.35 321.6 a* −2.9 −0.79 SiOxNy54 −3.95 −237.0 b* 0.376 −0.03 SiOx 80 −1.37 −109.6

In certain example embodiments the index-matching/stress-reducing layer5 may be adjacent to the glass substrate. In other example embodiments,layer 5 may be provided elsewhere in the stack.

FIG. 3 illustrates an exemplary example according to certain embodimentsof this invention. FIG. 3 is a cross-sectional view of an example coatedarticle according to an embodiment of this invention. The coated articleof the FIG. 3 embodiment includes substrate 1 that supports heattreatable (e.g., temperable) anti-reflection (AR) coating 4. Substrate 1is typically a glass substrate (e.g., clear, green, bronze, orblue-green glass substrate from about 1.0 to 10.0 mm thick), but may beother materials in certain example instances such as, for example, apolycarbonate or acrylic material, a silicon wafer, etc. The AR coating4 includes an “index-matching” and/or “stress-reducing” layer 5comprising silicon oxide, medium index layer 9 comprising siliconoxynitride, high index layer 9 comprising niobium oxide, and low indexlayer 11 comprising silicon oxide.

FIG. 4 illustrates a further example embodiment of this invention. FIG.4 is a cross-sectional view of a substrate supporting coatings on eachof its major surfaces. AR coating 4 includes, from the substrateoutwards, “index-matching” and/or “stress-reducing” layer 5, mediumindex layer 7, high index layer 9, and low index layer 11. AR coating 4′includes, from the substrate outwards, “index-matching” and/or“stress-reducing” Layer 5′, medium index layer 7′, high index layer 9′,and low index layer 11′.

It has advantageously been found that by seeking out high indexmaterials with a low post-heat treatment tensile stress, and/or byincreasing the magnitude of the contribution of compressive stress fromadditional layers in the stack, (e.g., a stress-reducing layer) it maybe possible to reduce the net tensile stress in a coating, and thusproduce a coating with an increased durability after heat treatment.

The layers described herein may be stoichiometric and/or substantiallyfully stoichiometric in certain example embodiments, whereas the layersmay be sub-stoichiometric in different example embodiments. However, itwill be appreciated any suitable stoichiometry may be used in connectionwith the any of the example layers described herein.

Although certain example embodiments have been described in connectionwith three-layer or four-layer AR coatings, other arrangementsincluding, for example, more or fewer layers, also are possible andcontemplated herein. For instance, certain example embodiments mayincorporate a stress-reducing layer, a medium index layer, and multiplehigh/low alternating layers thereon.

Furthermore, in some instances, other layer(s) below, within, or abovethe illustrated coating 4 may also be provided. Thus, while the layersystem or coating is “on” or “supported by” substrate 1 (directly orindirectly), other layer(s) may be provided therebetween. Thus, forexample, the coating 4 of FIGS. 2 and/or 3 and the layers thereof may beconsidered “on” and “supported by” the substrate 1 even if otherlayer(s) are provided between layer 5 and substrate 1. Moreover, certainlayers of the illustrated coating may be removed in certain embodiments,and other layers added in other embodiments of this invention withoutdeparting from the overall spirit of certain embodiments of thisinvention. In certain other example embodiments, coating 4 may consistessentially of layers 5, 7, 9 and 11, and layer 11 may be exposed to theatmosphere (e.g., layer 11 may be the outermost layer of the coating incertain example embodiments).

The example embodiments described herein may be used in connection witha variety of applications. For instance, a single-sided AR coating madeaccording to the example embodiments described herein may be used forapplications such as, for example, lights for commercial or residentialareas or at sports or other large venues or arenas, lighting applicationin general, touch screens, etc. A double-sided AR coating made accordingto the example embodiments described herein may be used for applicationssuch as, for example, electronics, displays, appliances, facades, etc.Of course, other applications also are possible for the exampleembodiments disclosed herein.

A coated article as described herein (e.g., see FIGS. 1-4) may or maynot be heat-treated (e.g., tempered) in certain example embodiments.Such tempering and/or heat treatment typically requires use oftemperature(s) of at least about 580 degrees C., more preferably of atleast about 600 degrees C. and still more preferably of at least 620degrees C. The terms “heat treatment” and “heat treating” as used hereinmean heating the article to a temperature sufficient to achieve thermaltempering and/or heat strengthening of the glass inclusive article. Thisdefinition includes, for example, heating a coated article in an oven orfurnace at a temperature of at least about 550 degrees C., morepreferably at least about 580 degrees C., more preferably at least about600 degrees C., more preferably at least about 620 degrees C., and mostpreferably at least about 650 degrees C. for a sufficient period toallow tempering and/or heat strengthening. This may be for at leastabout two minutes, or up to about 10 minutes, in certain exampleembodiments.

Some or all of the layers described herein may be disposed, directly orindirectly, on the substrate 1 via sputtering or other suitable filmformation technique such as, for example, combustion vapor deposition,combustion deposition, etc.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A method of making a heat treatable coatedarticle comprising an anti-reflection coating, the method comprising:disposing a stress-reducing layer over and contacting a glass substrate,the stress-reducing layer having an index of refraction thatsubstantially matches an index of refraction of the glass substrate;disposing a layer having a medium index of refraction over thestress-reducing layer; disposing a layer having a high index ofrefraction over the medium index layer; and disposing a layer having alow index of refraction over the high index layer, wherein the index ofrefraction of the high index layer is greater than those of the mediumand low index layers, and the index of refraction of the low index layeris less than that of the medium index layer; and wherein a net residualstress of the anti-reflection coating is compressive.
 2. The method ofclaim 1, further comprising heat treating the coated article with thestress-reducing layer, as well as the medium, high, and low index layersdisposed thereon.
 3. The method of claim 1, wherein the stress-reducinglayer comprises silicon oxynitride.
 4. The method of claim 1, whereinthe stress-reducing layer comprises silicon oxide.
 5. The method ofclaim 1, wherein the high index layer comprises an oxide of niobium. 6.The method of claim 1, wherein the high index layer comprises an oxideof titanium.
 7. The method of claim 1, wherein the high index layercomprises a thickness of from about 80 to 120 nm.
 8. The method of claim1, wherein a net stress value is approximated by the equation:${\sigma_{Net} = \frac{{\sigma_{1}h_{1}} + {\sigma_{2}h_{2}} + \ldots + {\sigma_{n}h_{n}}}{h_{1} + h_{2} + \ldots + h_{n}}},$where σ_(n) is the residual stress of the n^(th) layer, and h_(n) is thethickness of the n^(th) layer, and wherein a net residual stress σ_(Net)of the anti-reflection coating is predicted based on the residual stressand thickness of each layer in the coating.
 9. The method of claim 8,wherein the predicted net residual stress σ_(Net) of the anti-reflectioncoating differs from a measured net residual stress value by no morethan about 15%.
 10. The method of claim 8, wherein the predicted netresidual stress σ_(Net) of the anti-reflection coating differs from themeasured net residual stress value by no more than about 13%.
 11. Themethod of claim 1, wherein the stress reducing layer, medium indexlayer, and low index layer each comprise an oxide of silicon.
 12. Amethod of making a heat treatable coated article comprising ananti-reflection coating, the method comprising: disposing astress-reducing layer over and contacting a first major surface of aglass substrate, the stress-reducing layer comprising silicon, andhaving an index of refraction that differs from an index of refractionof the glass substrate differ by no more than about 0.2; disposing alayer comprising silicon oxynitride over the stress-reducing layer;disposing a layer comprising an oxide of niobium over the layercomprising silicon oxynitride; and disposing a layer comprising an oxideof silicon over the layer comprising an oxide of niobium, wherein theindex of refraction of the high index layer is greater than the indicesof refraction of the medium and low index layers, and the index ofrefraction of the low index layer is less than that of the medium indexlayer, and wherein a net residual stress of the coating is compressive.13. The method of claim 12, further comprising heat treating the coatedarticle after the layer comprising the oxide of silicon has beendisposed over the layer comprising an oxide of niobium.
 14. The methodof claim 12, wherein the stress-reducing layer comprises siliconoxynitride.
 15. The method of claim 12, wherein the stress-reducinglayer consists essentially of silicon oxide.
 16. The method of claim 12,wherein the stress-reducing layer and the layer comprising an oxide ofsilicon comprise approximately the same thickness.
 17. The method ofclaim 12, wherein the layer comprising an oxide of niobium comprises athickness of from about 80 to 120 nm.
 18. The method of claim 12,wherein a net stress value is determined by the equation:${\sigma_{Net} = \frac{{\sigma_{1}h_{1}} + {\sigma_{2}h_{2}} + \ldots + {\sigma_{n}h_{n}}}{h_{1} + h_{2} + \ldots + h_{n}}},$where σ_(n) is the residual stress of the n^(th) layer, and h_(n) is thethickness of the n^(th) layer, and wherein a net residual stress σ_(Net)of the anti-reflection coating is predicted based on the residual stressand thickness of each layer in the coating.
 19. The method of claim 18,wherein the predicted net residual stress σ_(Net) of the anti-reflectioncoating differs from a measured net residual stress value by no morethan about 15%.
 20. The method of claim 18, wherein the predicted netresidual stress σ_(Net) of the anti-reflection coating differs from themeasured net residual stress value by no more than about 13%.
 21. Themethod of claim 2, further comprising: disposing a secondstress-reducing layer over and contacting a second major surface of aglass substrate, the second stress-reducing layer comprising silicon,and having an index of refraction that differs from an index ofrefraction of the glass substrate differ by no more than about 0.2;disposing a second layer comprising silicon oxynitride over the secondstress-reducing layer; disposing a second layer comprising an oxide ofniobium over the second layer comprising silicon oxynitride; anddisposing a second layer comprising an oxide of silicon over the secondlayer comprising an oxide of niobium, wherein the index of refraction ofthe second high index layer is greater than the indices of refraction ofthe second medium and second low index layers, and the index ofrefraction of the second low index layer is less than that of the secondmedium index layer, and wherein a net residual stress of the coatingdisposed on the second major surface of the glass substrate iscompressive.